It relates in particular to improving the accuracy of the tightening, in particular improving the accuracy of the “pre-tension” force created when tightening a threaded fastener. This pre-tension is also commonly referred to as a preload or prestress. When carrying out an assembly using threaded fasteners, it is important to apply an appropriate pre-tension. This pre-tension makes it possible to hold the component parts of the assembly in contact and typically to compensate any additional forces caused by impact, vibration, pressure, sources of expansion or contraction—temperature or relative humidity variations.
A threaded fastener is generally a fastener employing a thread tightened by rotation or on which is mounted an internally threaded element tightened by rotation. This may notably mean a set screw screwed into an internally threaded hole, a nut-and-bolt or a nut tightened onto a stud.
The pre-tension is the force created on tightening a threaded fastener via the contact in the thread induced by the rotation of the nut or the bolt. In other words, this is the tension introduced into the assembly or the structure by tightening the threaded fastener.
Controlling the value of this force is notably important for the reliability over time of the assembly. Control herein means achieving a good accuracy of tightening, with a small spread around a target value.
In the prior art this problem is addressed via two distinct approaches. A first approach consists in optimizing the tightening techniques. For example, it is known to tighten a threaded fastener “by angle”, i.e. by applying to the element to be tightened a rotation by a predefined angle from a given initial position that tightens it.
It is also known to tighten a fastener of this kind “by torque”, i.e. by tightening until the tightening tool (for example a driver) reaches a predefined tightening torque.
Tightening can also be effected until it is determined that the elastic limit has been reached or to the onset of plastic deformation of the bolt, nut-and-bolt or stud.
It is also known to employ a so-called “tensioner” device replacing a nut on a bolt or a stud, the tensioner being a device of relatively complex construction enabling fine adjustment of the tension applied in the assembly.
These methods can be applied iteratively for greater precision, or even combined, for example by carrying out a pretightening by torque followed by a tightening by the required angle.
A second approach consists in instrumenting the assembly to determine the pre-tension applied. Various instruments are known for this purpose. Typically, a washer equipped with a piezo-electric sensor or a so-called preload indicating (PLI) washer may be inserted into the assembly. It is also known to employ tension indicator means marketed under the trade name Rotabolt®. Finally, dedicated devices may be employed to control the tightening, such as ultrasound devices, Bragg grating fiber sensors and strain gauges.
In the context of non-instrumented assemblies, corresponding to the least costly methods that are the quickest to use, typically in a industrial context, there is no method offering very good accuracy but quick to use.
For example, a simple bolt tightened by torque has a mediocre tightening accuracy, of the order of 30%, whereas a bolt tightened by torque and then by angle will have a slightly better tightening accuracy but takes longer to tighten.